The present invention relates to a current switch for monitoring low levels of electric current in a conductor.
Electrical devices that draw very limited current, often only a small fraction of an amp, are used in large numbers in many industrial and residential environments. The operation of these devices, such as small fan motors and heaters, can be important to the protection of valuable property or successful completion of a process that may involve costly or hazardous equipment or materials. These devices may be stand-alone devices controlled by a local controller, but are often components of an automated system that are monitored and operated by a remotely located controller or building management computer. Typically, the operation of the device is tracked by a monitoring device, such as a current switch, that includes by a current sensor that is electromagnetically coupled to a cable supplying power to the electrical device or load. The current sensor outputs a signal that is representative of the status of the current within the cable and, if the current changes significantly, a signal is transmitted to the controller. The controller may display a warning or an advisory signal on a control panel for a human operator and/or selectively enable or disable power to the load or another load(s) in response to the signal.
Holce et al., U.S. Pat. No. 5,808,846, incorporated herein by reference, disclose a protection device comprising a combination current sensor and relay for monitoring current in a cable supplying power to a load and controlling a device in response to a signal from a remotely located control panel. The protection device includes a sensing transformer comprising a wire wound core that encircles the power cable. A changing current in the power cable produces a changing electro-magnetic field around the cable which, in turn, induces a magnetic flux in the core of the sensing transformer. The magnetic flux in the core induces a voltage in the wire windings that is representative of the current flowing in the power cable. Thus, the power cable is the primary winding and the wire winding is the secondary winding of the sensing transformer. The wire winding is electrically connected to an input circuit that converts the voltage signal received from the secondary winding of the sensing transformer to an output signal representative of the current flowing in the power cable. The output signal is transmitted to a control panel and analyzed to determine if the controlled device is to be disabled or enabled. The control panel transmits an appropriate signal to a switch circuit, typically comprising a triac or relay, which responds to the signal from the control panel by shorting or isolating electrical terminals in series with the controlled device.
While the protection device disclosed by Holce et al. is compact and easy to install, it functions best with devices that draw substantial current. The current sensing input circuit is powered by energy sourced from the power cable through the wire winding of the sensing transformer. If the power cable current is too low, there may be insufficient energy to power the passive input circuit making current monitoring unreliable. For a protection device having a solid core sensing transformer and an input circuit of the type described by Holce et al., a minimum current of approximately 0.5 amps is required to generate sufficient flux to power the input circuit. The current draw of many electrical devices, including fractional horsepower motors, is insufficient for reliable detection with this type of sensing circuitry. What is desired, therefore, is a device for detecting very small currents flowing in electrical conductors.